VUV Spectroscopy of Atoms, Molecules and Surfaces

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The following three chapters are devoted to the spectroscopy of negative
ions which are particularly interesting from an astrophysical- and a theoretical
point of view. The structure, or energy-level diagram, of a negative ion is
markedly different from that of a neutral atom or -molecule due to the correlation
effects introduced by the presence of the extra electron. In general,
only a single bound state exists but the single-electron continuum may reveal
a rich structure of doubly-excited states, resonances, that are often associated
with excited states of the corresponding neutral atom or -molecule. A
doubly-excited state typically decays by electron emission in a process known
as autodetachment, leaving the resulting neutral system in its ground state or
a lower-lying excited state. In chapter 2 a general introduction to the properties
of negative ions is given, including an overview of the present knowledge
obtained from spectroscopic studies and the theoretical approaches currently
applied to model their structure.
An experimental technique for Doppler-tuned VUV spectroscopy of highlying
doubly-excited states of the fundamental negative-ion system H− ,has
previously been implemented at the storage ring ASTRID. In these studies
use was made of a fixed-frequency VUV light source based on the non-linear
frequency-upconversion of visible laser light by a process known as harmonic
generation. The unprecedented spectral resolution achieved with this technique
has allowed the observation of previously un-resolved resonances and
has recently been improved by the application of the electron cooler of the
storage ring. This has allowed the first precise measurement of the spectral
width of one of these resonances, providing the first critical test of a number
of theoretical models as described in chapter 3.
The ASTRID storage ring has also previously been successfully used for
lifetime measurements of metastable, doubly-excited, autodetaching states of
atomic negative ions with lifetimes in the µs–ms range. The spectral widths
of such states are too narrow that they can be resolved by ordinary laserspectroscopic
methods. In the storage ring the ions are allowed to circulate
for several lifetimes, facilitating a precise measurement of the decay rate as a
function of time. In the experiments described in chapter 4this method has
been applied to lifetime measurements on unknown metastable states of N − 2
and CO − 2
which do not exist as stable ground-state ions. By a co-operation
with theorists, the lifetimes observed have been assigned to a previously unexplored
class of high-spin negative-ion states.
The focus of chapter 5 is on the construction of an experimental setup aiming
at studies of structural dynamical processes at surfaces with a femtosecond
time resolution. Emphasis is given to chemical reactions of gas-phase
molecules which are enhanced, or catalyzed, by the presence of the surface,
in particular to the desorption step, by which the product molecules escape